What are B mode and E mode signals?

I've read an article in nature and found technical words "B mode signal" and "E mode signal". I don't understand what they are. In a website, they say it's like the electromagnetic field line but I'm still not clear.

I also don't understand what k means in " the linear greyscale shows the E-mode lensing convergence field k(Kappa)". I think it's also the technial word.

I do not know the details but this is known as E-B mode decomposition. Basically the E-mode is a measure of the stretching (shear) of the light source that is weak lensed. This mode is curl free and is generated by gravitational effects. As far as I know the other mode maps, the B-mode maps, are the noise components that cannot be identified with the gravitational physics of the lens.